View Full Version : What are the signs of a bad IAC?

July 11th, 2006, 08:58
I started my 88 cherokee the other morning, and the idle went to 2800rpm's. Waited five seconds and it came back down to normal. I tried this a few more times hoping for a fluke and it still happened again. I figured it was the IAC. The entire TB is cleaned including iac pintle and port. I unplugged the iac, took it out, pulled the pintle out (extended it) put it back in, leaving it unplugged and it idles fine now. But if i plug the connector back in, it will start revving funny again.

By the way...88 cherokee 4.0 AW4 RENIX. Any help is great. I would like to know this for sure before i spend $44 on a new one. Thanks.

July 12th, 2006, 05:47

Rick Anderson
July 12th, 2006, 05:53
I'm not familiar with the Renix system. The Chrysler system will throw a code for a bad IAC, or what the computer sees as a bad IAC. Of course a shorted wire to the IAC could cause a perfectly good IAC not to work right and the computer to see it as a bad IAC.

It certainly sounds like a bad IAC from your troubleshooting, I'd just check the resistance with a multi-meter. If its zero, then you know its bad, you need to know the spec if its a low resistance, because motors/servos/soldenoids usually have a very low resistance normally when you check them static with a multi-meter.

July 12th, 2006, 19:59
Unplug your battery and plug back in the IAC, it has a memory and its just stuck, the computer will relearn the where the idle should be. At least thats the easy fix that worked for me. It happened to me after a real intense cleaning of the TB. Same thing happened. And once I put the battery cable back on it did it once(high rev.), went back to right idle. and has stayed there every start since. ( I have to look but the iac actually is controlled by a relay I believe its the latch relay, but taking the battery out of the mix for a while clears it) Good Luck

July 12th, 2006, 20:47
generally when an Iac fails it will fail in the closed position, so you will have symptoms like hard/no start unless you put foot on throttle, or maybe starting and dying immediatly, dying when coming to a stop etc. Disconnecting you battery for 10 minutes and leaving a door open will clear the Keep alive memory in the Pcm. KAM are the adaptive strategies your jeep has learned to run efficiantly. If your jeep has learned that it has to command the iac open a certain percentage to maintain its idle spec and then you clean everything out changing that amount. then you could get a high idle until you reset that memory.

Forgot to mention I am not sure if the renix system has a keep alive memory, its common on newer vehicles, but I dont know about in 88 you might also want to look at your TPS, lots o problems with those.

July 13th, 2006, 19:34
On my 1990 with the same Renix system, a high idle has always been a faulty TPS. I'm on my third since new. The intermittent nature of the problem, dropping back to normal idle speed after 5 seconds, is alo what mine did. I could sometimes get the idle back down by gassing it very briefly. Can't advise on the IAC, mine's OK.

July 13th, 2006, 22:47
The renix doesn't have a "kam"....

July 13th, 2006, 23:01
No keep alive memory, no DTCs on RENIX.

I'm having a similar problem, and my TPS is a month old and tests good. (88XJ/4.0/BA-10/D30/D35/3.07)

I'm thinking TPS, since it's an intermittent high idle that sometimes sorts itself out, and sometimes sorts itself out with a brief "power-cycling." When the idle jumps up to 3Krpm, I get a little annoyed (especially at $3+ per gallon!)

I think I've got a spare IAC, and I'll let you know how that works out for me once I get it replaced...


July 14th, 2006, 12:28
I too have the same problem(high idle upon start up) and the tps tests ok.I'm wondering if the latch circuit has anything to do with the high idle at start up....My theory is if the latch circuit isn't sending the iac proper input after shut down this will cause high idle because the iac wasn't told what position to be in....